Right here's the deal.
Until Samsung get off their lazy behinds and release 5.1.1 firmware and the source code for the LTE models nothing can be done.
So if you want root on 5.1.1 lte models then you have to use the t810 version and lose lte.
This is because a modified kernel is needed to allow rooting on 5.1.1 as samsungs stock kernel blocks it.
Eventually the source code and firmware will be released and you can then reflash back to stock if you so wish.
If you want to unroot before this time then all you need do is make a full backup with TWRP *BEFORE* rooting.
To return to stock restore the backup.
When the source code for the t815 LTE model is released then a modified kernel can be compiled and LTE will be working as normal.
Considering the t815 kernel will be near identical to the t817 model then it should work for that model also.
Samsung have a habit of violating the GPL and need badgering to release the source code. They should have released the source code for the T817 5.1.1 kernel already as it's being shipped with those devices.
So if you want it badger them.